The present invention relates to specimen holders for transmission electron microscopes, and more particularly to side-entry specimen holders which are tiltable about two axes and rotatable about the electron beam axis.
In recent years, analytical specimen holders used in transmission electron microscopy (TEM) have become more widely used because of their simple and reliable mechanisms; however, as the technology of new materials advances so do requirements for TEM specimen viewing and specimen positioning. Conventional side entry TEM specimen holders have been developed which are capable of only a single axis tilt, which is the simplest type of specimen holder. In this form, the specimen is fixed to the specimen holder tip and has no means of independent motion. Tilting of the specimen in this axis (i.e., the longitudinal axis of the specimen holder) is accomplished by tilting the complete specimen holder about this axis in either direction. As technology has improved, the need for a second axis tilt (i.e., the axis perpendicular to the longitudinal axis of the specimen holder) of the specimen in addition to the first axis tilt became apparent and led to the development of double tilt specimen holders which are commercially available today. Double tilt specimen holders combine the two tilt axes as described above with the second axis tilt in either direction typically accomplished by movement of a drive rod mechanically coupled to a cradle supported by two pivot pins (axles).
TEM studies involving selected-area diffraction, stereo imaging, diffraction-contrast analysis, domain structure analysis and electron tomography all require precise of the specimen orientation. Single-tilt and double tilt holders are normally inadequate for these applications since the specimen orientation requires both tilting and rotation. Planar features such as interfaces and grain boundaries within a TEM specimen are most easily studied and analyzed if the specimen is first rotated until the planar feature is aligned parallel to the main tilt axis of the specimen holder. Such single tilt and rotate holders are also commercially available such as the Model 650 Single Tilt Rotate Specimen Holder available from Gatan, Inc. of Pleasanton, Calif. Stereological studies of crystalline materials have been hindered in that single tilt and rotate holders were limiting in use due to lack of a second axis tilt mechanism. Such stereological studies are complicated by the contrast change that occurs when a specimen is tilted. To minimize this effect, the specimen can first be rotated so that the g vector principally responsible for the contrast is aligned parallel to the tilt axis. The specimen can then be tilted without substantially altering the diffraction-contrast conditions. However, this involves manually changing the position of the specimen while outside of the transmission electron microscope (i.e., requires specimen unloading and re-loading) to attain proper alignment. The ability to tilt in two axes and rotate a specimen containing a feature (interface) to align the feature in the direction of various microscope detectors can facilitate feature analysis in the TEM. Moreover, the use of CCD cameras for TEM digital imaging makes it desirable to be able to rotate the specimen in a specific direction (visual image alignment) for a given crystallographic condition.
Thus, there remains a need in this art for a specimen holder which is able not only to rotate a specimen to obtain any desirable direction, but also to be able to tilt such a specimen in two tilt axes while viewing the specimen in a TEM.
The present invention meets that need by providing a side-entry specimen holder for transmission electron microscopy capable of rotating a specimen and tilting it in two axes. The specimen, when mounted in the holder, can be tilted in the plus/minus direction of the first (longitudinal) axis, the plus/minus direction of the second (normal to longitudinal) axis, and simultaneously have the ability of continuous 360xc2x0 rotation in the axis of the electron beam to permit alignment while viewing the specimen in the TEM. Further, because of the provision for rotation of the specimen, the viewing axis of the specimen may be aligned to the first or second axis, permitting very high tilt angles.
In accordance with one aspect of the present invention, a side-entry specimen holder for a transmission electron microscope is provided and includes a specimen holder having a specimen cradle, a cradle frame, and a drive shaft connected to the specimen cradle and frame; a first tilt mechanism for tilting the specimen holder about a first longitudinal axis; a second tilt mechanism for tilting the specimen cradle about a second axis normal to the first axis; and a rotation mechanism for rotating the specimen cradle. In a preferred form, tilting in the first axis is accomplished by tilting the entire specimen holder. A preferred first tilt mechanism comprises a drive for rotating the support arm about its longitudinal axis. The second tilt mechanism comprises a drive for moving a drive shaft which is linked through a spherical bearing to a frame housing the specimen cradle. The frame for the specimen cradle also preferably includes a support arm (barrel).
The rotation mechanism may be any of a number of mechanisms which are described below and may be endless (full 360xc2x0) or limited in rotation. However, a preferred rotation mechanism comprises a pinion drive gear mating with a ring gear. The specimen cradle includes the ring gear about its periphery, which is engaged and driven by the pinion gear to provide for the rotation of the cradle.
In accordance with another aspect of the invention, a method for analyzing one or more physical features of a specimen in a transmission electron microscope is provided and includes the steps of mounting a specimen to be analyzed in a holder capable of tilting in two axes and rotation, inserting the holder and specimen into the path of an electron beam from the transmission electron microscope, rotating the specimen in the holder to align a physical feature of interest relative to a first tilt axis, and tilting the holder in both of the first and second axes to analyze said physical feature. The physical feature of interest may include microstructural features selected from the group consisting of grain boundaries, precipitates, and interfaces in crystalline materials. Optionally, the method includes the step of analyzing the specimen when tilted about the first axis, rotating the specimen in the holder while in the transmission electron microscope to align the physical feature of the specimen to the second axis, tilting the specimen about the second axis, and performing further analysis of the specimen. Another aspect of the invention provides a method for analyzing one or more physical features of a specimen in a transmission electron microscope and includes the steps of mounting a specimen to be analyzed in a holder capable of tilting in two axes and rotation, inserting the holder and specimen into the path of an electron beam from the transmission electron microscope (TEM), rotating the specimen in the holder to align a physical feature of interest with a TEM detector, and tilting the holder in both of the first and second axes to analyze the physical feature.
Accordingly, it is a feature of the present invention to provide a side-entry specimen holder for transmission electron microscopy capable of rotating a specimen and tilting it in two axes. This, and other features and advantages of the present invention, will become apparent from the following detailed description and the accompanying drawings.